1. Field of the Invention
The present invention relates generally to inspection of turbine blades and, more particularly, to an optical inspection system for visually inspecting turbine blades during the turning gear operation. The invention also relates to a method of visually inspecting turbine blades at turning gear.
2. Background Information
Gas and steam turbines for electrical power generation are very expensive (i.e., tens of millions of dollars each). For maximum efficiency, they should not be removed from service for inspection or maintenance unless absolutely necessary. However, defects in the blades of large turbines can cause serious damage and possibly injury. It is, therefore, necessary to be able to promptly detect the formation of blade defects.
Reliable and early detection of failures that could be catastrophic to the power generation unit and the ability to bring the unit through the critical period until the next planned outage, are paramount in keeping repair costs low and guaranteeing long term safe operation. Evaluating the condition of the turbine blade thermal barrier coating (TBC), commonly referred to in the art as TBC-monitoring, is the first step of analyzing the status of the turbine blades. In modern, high-performance gas turbines, for example, TBC-monitoring is necessary to ensure the integrity of the blades.
Known conventional methods of inspecting turbine blades such as surface inspection methods (i.e., magnetic particle testing; eddy current testing; dye penetrant techniques) and volumetric methods (i.e., ultrasonic testing) rely on the periodic disassembly of the turbine. Disassembling a turbine to inspect it is an expensive process and takes the turbine out of service for a significant amount of time. Unfortunately, none of the foregoing techniques are suitable for inspection while the turbine is on-line and running under load. More recent turbine inspection techniques employ a variety of apparatus and methodology in an attempt to offer on-line TBC-monitoring for full load operation.
For example, U.S. Pat. No. 4,380,172 entitled, “On-line Rotor Crack Detection,” discloses a method of detecting incipient cracks in the rotor of a fluid powered turbine while the turbine is on-line and running under substantially normal load. Vibrations in the rotor are monitored and a signature analysis of normal vibration patterns is performed in order to establish a vibration spectrum for purposes of comparison. The turbine is then perturbed, for example, by changing the temperature of the motive fluid (i.e., changing the temperature of steam in a steam driven turbine), and the signature analysis is again performed to determine changes in the vibration pattern. An increase in the amplitude of the fundamental frequency and the appearance and increase in amplitude of higher harmonics following perturbation indicates the presence of a defect in the rotor.
U.S. Pat. No. 4,685,335, entitled “Method and Apparatus for Monitoring Cracks of a Rotatable Body,” discloses the use of acoustic emissions (AE) signals in order to detect turbine blade cracks. Discovery and evaluation of the crack (i.e., depth of the crack) are accomplished by comparing the AE signals with assumed vibrations of the rotatable body. The method permits discovery of cracking in the rotatable body from its inception and also the progress of cracking on an on-line basis.
U.S. Pat. No. 4,955,269, entitled “Turbine Blade Fatigue Monitor” discloses the use of passive proximity probes to inspect turbine blades. Specifically, an on-line vibratory fatigue monitor measures displacement of the blade to generate a displacement signal and calculate accumulated fatigue in the blade based thereon. The method requires constant monitoring of vibratory displacement and changes in the steady state stress.
U.S. Pat. No. 5,670,879, entitled “Nondestructive Inspection Device and Method for Monitoring Defects Inside a Turbine Engine,” discloses another method of monitoring a defective condition in a rotating member of a combustion turbine. The method uses a holder assembly to position an ultrasound transducer or eddy current sensor near the rotating member without disassembling the turbine. Signals indicative of the monitored condition are recorded and compared to a signal representation generated from a reference standard having a known defect so that a defective condition can be discovered.
Unfortunately, each of the foregoing inspection methods and apparatus has its own unique set of disadvantages. The interior environment of a turbine is an extremely hostile environment for electrical equipment (e.g., without limitation, cameras; sensors; illuminating equipment). For example, a gas turbine typically operates at an internal temperature of about 1200° C. (2192° F.) near the “row 1” blades, while a steam turbine can have temperatures of up to about 550° C. (1022° F.). The current state of electronics technology is limited to temperatures well below this. Therefore, working on-line with pyrometers and/or infra red (IR) technology or with one of the other aforementioned apparatus, for example, in such environments requires significant effort in terms of cooling. Generally, 200° C. (392° F.) is considered to be the maximum practical temperature for operating electrical equipment. High pressures and reactive chemistry within turbines provide further detriment to inspection and measurement equipment. Accordingly, it will be appreciated that the foregoing apparatus (e.g., without limitation, IR cameras) and procedures required for on-line turbine blade inspection are very cost intensive.
There is a need, therefore, for a reliable and cost-efficient method and apparatus for visually inspecting turbine blades.
Accordingly, there is room for improvement in the art of turbine blade inspection.